1987 K75S Gearbox Rebuild Due to Inability to Downshift

[jay1622]

Good evening my fellow Bricksters! I'm back. Good news is that the wife told me I should keep the rebuild; seeing the effort and what it represents (those of you in the know, know the story). Bad news is this... Remember that trip to the Dragon? The inaugural ride? Remember me saying I dished the gearbox in some fashion? Well, she hasn't come back, and only continues to get worse.

Symptoms:
Upshifts beautifully when preloaded and then with the clutch applied. No issues there.
Downshifts are impossible. Non-existent. They ain't happening folks.

The only way the bike will downshift is if I blip the throttle and sync the gears (duh), or if I'm at a complete stop and I rock the bike forward and back. I've read forum thread after forum thread and have narrowed the culprit down to the shifting forks, the fork axles, the shifting fork roller pins (cylinderical rollers), the shifting roll, the step plate (doubtful), or the pawl. Am I missing anything? I've ordered a replacement gearbox from EBay de jour. The input and output splines looked great, the gear selector switch looked brand new (mistreated ones tend to disintegrate), there wasn't any bumps, dents, scratches or anything, and the donor bike is reported to have had 36k miles on it.

When I take the two gearboxes apart (I have decided to do this to inspect the donor gearbox) what should I be looking for. The shifting axles can be visually inspected and easily checked for warpage. The shifting fork contact points will be obvious, but is there anything else on those I should be looking for? The roller pins are known to break apart, so that'll be obvious. Shifting rolls are fairly tough, but I'm guessing damage/wear will stand-out. The step plate? I just can't imagine anything will be wrong with that. I'll check the pawl for warpage. The only other thing I can think of is the selector shaft or the rod for the pawl.

Am I missing anything gents? The bike is still together, so I'm not in a position to take pics; though, I will when I start tearing into it.

As always, thanks!

[The Mighty Gryphon]

Is there a lot of free play in the shifter?  When I bought my K75RT I had some problems with downshifts because the infamous grub screw was loose and there was so much free play that it was difficult to push it down enough to change gears.  Upshifting was okay because there is a lot more room to lift the shifter than to push it down. 

When you open up the transmission that is the first thing I would look at. 

[Laitch]

When I bought my K75RT I had some problems with downshifts because the infamous grub screw was loose . . .   
When you open up the transmission that is the first thing I would look at.

Here's an introductory video. In this video, the action starts at 1:00.



There are a couple of important observations in this thread, too.

[jay1622]

It feels tight, and I don't believe there's anymore play than there was before, but I'll certainly give it priority when I open the case. Thanks guys. Great link too by the way; did quite a few searches, and that one didn't come up.

[wally.fisher]

When did you last lube the gearbox spline shaft?.


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[jay1622]

About 11 months/3,000 miles ago. Good question.

[gone_ape]

I happened to come across this over at the BMOA forum...thought it interesting and relevant...
It was an old discussion on interchangeability transmission issues with a K100...
Food for thought.....


https://forums.bmwmoa.org/showthread.php?59459-1986-K100RS-Transmission-Interoperability


QUOTES FROM THE BLOG ENTRY LINK ABOVE:
 
"ANY K75 transmission will fit - and they are identical to yours. I'd avoid '85 and probably early '86 transmissions since they had a soft shifter drum which eventually wears and causes the transmission to lock into a gear (or not be able to get into one..) Lower mileage is usually better - and ask for a good closeup photo of the input shaft splines on the used transmission - and post it here so we can comment on wear (or hopefully - lack of wear.)"


"Don, I think that even some of the 87's had the soft shift drum issue. I know mine did and refused it up shift on a regular basis. After purchasing a 88 K100 RS gear box, I disassembled the K75 unit and the only problem with it was the self imposed detent in the drum. Thank goodness for a infinite supply of cheap parts."

[beemrdon]

Unless you are viewing this as some kind of a project, I would just go and buy a used one on ebay for a few hundred. :2thumbup:

[Motorhobo]

Unless you are viewing this as some kind of a project, I would just go and buy a used one on ebay for a few hundred. :2thumbup:

+1  I got one a couple months ago for $250 incl shipping from some guy in Oregon. Works great.

[rbm]

ANY K75 transmission will fit - and they are identical to yours. I'd avoid '85 and probably early '86 transmissions since they had a soft shifter drum which eventually wears and causes the transmission to lock into a gear (or not be able to get into one..) Lower mileage is usually better - and ask for a good closeup photo of the input shaft splines on the used transmission - and post it here so we can comment on wear (or hopefully - lack of wear.)

AFAIK, any K75 Monolever, K100 Monolever, K1100 Paralever and classic K1200 Paralever is bolt up compatible with the K75/K100 engine block.  It is possible for the monolever transmissions to bolt up to the original bell housing.  In the Paralever case, it is better to replace the original bell housing with the replacement bell housing, especially with the K1200 because one has to adapt the hydraulic clutch (no cable operated clutch).  I know this is possible because I own a K75 with a K1100 rear end and I've seen a K75 with a K1200 rear end.

[jay1622]

Ca'mon guys! Haven't you figured me out by now? Do you really think I'm going to install a donor gearbox without tearing into it and inspecting it first?  :hehehe


On a serious note, my $220 (delivered) 1987 K75 with only 36,584 mi on the clock arrived today.


Pro's: Splines are pristine, the gear selector switch thing (the sending switch with the housing that always disintegrates) looked new, there's no leaks, and (best of all) the shifting forks and shifting roll look brand new.


Con's: The P/O had a love affair with moly-b, and boy what a mess!!!

[Martin]

Jay coat the gear selector switch thing with Plasti Dip or something similar. Coating it stops the water and road crud from entering which will prolong it's life. I did mine about 18 years ago haven't had a problem since, I cleaned and coated the old one which I now have as a spare.
Regards Martin.

[gone_ape]

I would have to say "Scoreboard"!!  Nice find.

Something I found odd when I bought my  'replacement' tranny.....It was said to have come from a 84 K100RS..and I automatically assumed everything was straight swap interchangeable with my 94 K75 ..... the gear indicator switch was identical (brown plug, same shape,  etc) as were the all of the visual stuff ....save one item.....the clutch arm (part number 7 below) was different.  Let it be known I was pre-aware of having to change the clutch throwout bearing and the clutch push rod to suit my k75 application....


And since it was poorly packaged, during shipment, it arrived bent.  This is when the difference was noted.

My clutch arm off the 94 transmission and the 84 K100 donor were (are) different part numbers, machined differently and therefore were not interchangeable...and as such the trans cases are slightly different where/how the clutch arm mounts.... something that has not been discussed (at least from my feeble research)....so, something to look for....

That said, I noticed no difference in replacement seals, mounting the trans to the engine nor in it's operation, but my bent clutch arm required purchasing that particular part that was for that series (84-89 K100RS) of transmission.

[wally.fisher]

 So what's the plan then? New bearings or just to look?


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[Martin]

Part no 7 benefits from installing a grease nipple. The occasional shot of grease keeps it lubed and stops water  and crap from entering. I also occasionally lube the clutch cable with a PTFE lubricant called Triflow which does not degrade the Teflon liner as does some other lubricants. Triflow is sold in bicycle shops for chain and cable lubrication. These modifications along with lubing the lever ball ends I believe have allowed my Brick to be still running the original 25 year old cable.
Regards Martin.

[jay1622]

Jay coat the gear selector switch thing with Plasti Dip or something similar. Coating it stops the water and road crud from entering which will prolong it's life. I did mine about 18 years ago haven't had a problem since, I cleaned and coated the old one which I now have as a spare.
Regards Martin.

Very very good call here and on the more recent post. Besides longevity, anything to smoothen and reduce the pull is always welcome.

[gone_ape]

This is what of Martin speaks... sooo easy to modify a caveman can do it....*shameless PR campaign rip*

[jay1622]

For those of you in the know, please see the vid and tell me what you think. The grub screw on the shift axle was tight, the shifting fork axles are true, and all else (so far) looks good. While I still need to compare and measure the shifting forks, I foresee myself just using the donor forks. I'm picturing the function of the gearbox in my head while downshifting, and can't figure out why a loose step plate would be the culprit.

[gone_ape]

There's a roller that engages that star wheel (on a spring loaded arm inside the front transmission cover) so it seems to me if that roller/arm was in good shape, AND it was correctly indexed, it would lock the shift drum in it's position once a gear change was selected....but with that pin for the star wheel loose along with the possibility of worn fork peaks/valleys in the shift drum, the fork would have movement not intended -- the gears would not fully engage and this would wear the gear dogs and slots prematurely....Do the sliding gear dogs and corresponding slots look sharp and not rounded (hammered) at the points of engagement??
 
Not to sound like an idiot, (subjective), but I have a  Stupid Question of the Day: when the trans was put back together was the procedure followed for that roller arm prior to reassembly?  i.e., holding the arm back against spring tension, , inserting a rod through the front cover to hold the roller arm away from the shift drum's star wheel, replacing trans front cover, then releasing the roller arm??


Although it's not really clear in the vid, what do the channels in the shift drum look like??  Are they well worn at the "peaks" in the channels where the shift fork rollers go? If these are worn, the forks would have difficulty sliding smoothly in those channels possibly hanging up a shift or two by preventing the fork to slide as it should........


Maybe there's a two fold problem here.  The star wheel is loose on the drum and the shift drum's fork channels are worn down and the slop soup is manifesting itself in this way .....gears not fully engaging/disengaging

[jay1622]

There's a roller that ...

Good morning. As you will see in the video, the roller is in great shape, as there is no bending/warping and the spring tension is identical to the donor gearbox (strong and smooth). In the video, I only give one angle of one section of wear on the shifting roll (drum/cylinder thing), but aside from wear, the only only sign of damage on the suspect shifting roll is the step plate pin hole.


Never a stupid question bro... I have yet to fully reassemble either of the gear boxes, but I did do a test run in the suspect case. After verifying the selector shaft was true, the grub screw was set and tight, and all else was true and functional, I inserted both the donor and suspect selector drums in the case and installed the case cover. There was a significant difference in the accuracy and immediacy of shifting between the two.


I've tried to measure and compare the suspect shifting forks to the donor shifting forks, and I don't see a difference. As I've mentioned previously, I'm going to just use the donor forks for good measure.


The video adds to my response for ya...

[jay1622]

Bad Shifting Roll Test...

[jay1622]

Good Shift Roll Test...

[jay1622]

Just an FYI video for anyone who's never seen how the selector shaft engages the roll...

[gone_ape]

Great vids....I'm leaning towards that loose star wheel/pin on your original shift drum not allowing the gears to engage/disengage properly.


 Try putting the trans back together without the front cover to test shift fork ease of movement.  You'll have to spin the input and or output shafts "as you shift" as they won't move properly just sitting static.  This way you can observe the gear dogs as they engage in their respective slotted wheels.  I'm thinking the 'slop' created by that shift drum issue is not allowing the gears to fully move/engage/disengage.  Check those dogs/slots and report back!!!

[jay1622]

The newer gear boxes are tight enough to be able to function-test without the case cover; however, with the k-bike input shafts having tapered roller bearings and relatively loose shift rolls, it's dang-near impossible. I gave both the output shaft and the intermediate shaft a thorough inspection, and everything was smooth as silk. Now that I think about it, that step-plate retaining pin being loose would lead to the symptoms I was experiencing; as you (gone-ape) suggested - good call early on.

I reassembled the gearbox last night and performed a successful function check. All of the seals have been ordered, but there's one that's back-ordered until early 2018. I may have to source that one elsewhere.

I'm looking forward to getting her back on the road, for sure.